This invention relates to a method and apparatus for controlling the air-fuel ratio demanded by a fuel controller in order to maintain optimum performance of a catalytic converter.
Three way catalytic converters are used to reduce exhaust emission of nitrous oxides (NOx) hydrocarbon (HC) and carbon monoxide (CO). In a steady sate of operation the performance of the catalyst in removing these pollutants is at an optimum level when the air fuel ratio of the exhaust gas is within a narrow range, close to the stoichiometric air-fuel ratio.
Conventionally, a fuel controller is used to control the air fuel ratio demand from an engine based on feedback from an air fuel ratio sensor upstream of a catalytic converter in the exhaust passage. In other known control systems two air-fuel ratio sensors are used, one upstream of the catalytic converter, and one downstream of the catalytic converter.
In one example of such fuel control schemes, the air fuel ratio demand is increased until the air fuel ratio sensor detects that the ratio demand is too lean, whereupon the requested air fuel ratio is switched to request the stoichiometric air fuel ratio, and gradually decreased until the air fuel ratio sensor detects that the ratio demand is too rich. In other examples, when the air fuel ratio becomes too rich or too lean the requested air fuel ratio is switched to request an air fuel ratio which is half way between the maximum and minimum air fuel ratios which have caused previous switching.
This invention provides a method and apparatus for operating an improved fuel control scheme in which exhaust emission of pollutants are reduced.
According to the present invention there is provided a fuel control system comprising a first sensor arranged to measure an air fuel ratio upstream of a catalyst; a second sensor arranged to measure an air fuel ratio downstream of the catalyst; a catalyst model arranged to determine oxygen storage characteristics of the catalyst; a catalyst model arranged to estimate an oxygen storage value of the catalyst in dependence upon the measured air fuel ratio upstream of the catalyst, upon the measured air fuel ratio downstream of the catalyst and upon the determined oxygen storage characteristics of the catalyst; a controller for requesting an air fuel ratio according to a switching value derived from the estimated oxygen storage value of the catalyst, in which the controller is arranged to perform the following steps
a) request a maximum ratio of air to fuel when the switching value is less than a first threshold;
b) gradually decrease the requested air to fuel ratio from said maximum ratio;
c) request a minimum ratio of air to fuel when the switching value is greater than a second threshold; and
d) gradually increase the requested air to fuel ratio from said minimum ratio.
There is a time delay between changing the air fuel ratio demand, and the resulting change in the estimated oxygen storage value which means that if the air fuel ratio demand is changed due to the estimated oxygen storage value reaching a predetermined threshold then that predetermined threshold will be exceeded, or xe2x80x98overshotxe2x80x99, due to the time delay. To alleviate the problem of the time delay, in a preferred embodiment a future oxygen storage value is predicted. Accordingly the fuel control system further comprises an oxygen storage predictor arranged to perform the following steps:
estimate a future oxygen storage value of the catalyst (2) in dependence upon the estimated oxygen storage value, the determined oxygen storage characteristics and a requested air fuel ratio; and
derive said switching value from the estimated future oxygen storage value.
As the catalyst ages the engine has to be run leaner to achieve a predetermined level of oxygen storage in the catalyst, and has to be run richer to achieve a predetermined level of oxygen depletion. To alleviate the problem of changing characteristics as the catalyst ages, in another embodiment of the invention, instead of controlling air fuel ratio demand using a predetermined estimated oxygen storage threshold the air fuel ratio may be controlled taking into account characteristics of the catalyst which are modelled by the catalyst model.
Therefore the oxygen storage predictor is arranged to derive said switching value from the estimated future oxygen storage value in dependence upon said oxygen storage characteristics.
Advantageously, the rate of decrease is dependant upon the difference between the switching value and the first threshold and the rate of increase is dependent upon the difference between the switching value and the second threshold.
According to another aspect of the present invention there is also provided a method of requesting an air fuel ratio according to a switching value derived from an estimated oxygen storage value of a catalyst comprising the steps of
a) requesting a maximum ratio of air to fuel when the switching value is less than a first threshold;
b) gradually decreasing the requested air to fuel ratio from said maximum ratio;
c) requesting a minimum ratio of air to fuel when the switching value is greater than a second threshold; and
d) gradually increasing the requested air to fuel ratio from said minimum ratio.
In a preferred embodiment said switching value is derived from an estimated future oxygen storage value, in which the estimated future oxygen storage value of the catalyst is estimated in dependence upon the estimated oxygen storage value, determined oxygen storage characteristics and a requested air fuel ratio
Preferably said switching value is derived from the estimated future oxygen storage value in dependence upon said oxygen storage characteristics.
Advantageously, the rate of decrease is dependant upon the difference between the switching value and the first threshold and the rate of increase is dependent upon the difference between the switching value and the second threshold.